1. Field of the Invention
The present invention relates to a sheet conveying roller and a sheet feeding apparatus.
2. Description of the Related Art
Conventionally, image forming apparatuses such as printers, copier or facsimiles, or image reading apparatuses are provided with a sheet feeding apparatus to separate a plurality of recording papers or documents (hereinafter, simply referred to as a sheet) that is stacked in a sheet storage unit and feed out the separated sheets to an image forming section or an image reading unit on a one-by-one basis.
The sheet feeding apparatus includes a sheet separation unit for separating sheets one by one. Japanese Patent Application Laid-Open No. H07-301248 discusses a structure employing a separation roller as the sheet separation unit. The separation roller uses a separation pad and functions as a torque limiter. In such a sheet separation unit, a separation roller having a torque limiter connected on the same shaft or incorporated therein is brought into pressure-contact with a feed roller, separating sheets by the braking torque of the torque limiter.
For example, when only one sheet is nipped by the feed roller and the separation roller, a large rotation torque is applied to the torque limiter, allowing the separation roller to rotate following the rotation of the feed roller. Such rotation of the separation roller following the rotation of the feed roller will be referred to as accompanied rotation. On the other hand, when a plurality of sheets is placed between the feed roller and the separation roller, a relatively small rotation torque is applied to the torque limiter, suppressing accompanied rotation of the separation roller with the feed roller. In this way, by suppressing the accompanied rotation of the separation roller with the feed roller, the feed roller can convey only one sheet at a time while the separation roller prevents two or more sheets from being conveyed at the same time.
That is, when a plurality of sheets is nipped by the feed roller and the separation roller, the braking torque of the torque limiter decreases to a lower limit so as to suppress the accompanied rotation. On the other hand, when only one sheet is placed between the feed roller and the separation roller, the braking torque of the torque limiter increases to an upper limit so as to allow the accompanied rotation. By controlling the braking torque within the above-described range, the sheet separation function and the sheet feeding capability can be properly provided.
Such a structure is known to be capable of provide a stable sheet feeding operation while maintaining excellent durability and preventing the pad and sheet from making a fluttering sound compared with a structure having a sheet separation unit employing a separation pad. A typical torque limiter usable in such a sheet feeding apparatus is equipped with powder clutches or brakes and a coil spring.
As illustrated in FIG. 11, the conventional sheet feeding apparatus includes a separation roller 16a connected to a torque limiter 16b so as to be rotatably held on a separation roller support member 16c along with the torque limiter 16b. Referring to FIG. 11, the separation roller 16a is brought into pressure contact with a feed roller (not illustrated) by means of a spring 16d. 
Connection between the separation roller 16a and the torque limiter 16b is not limited to such a manner. As illustrated in FIG. 12A, the torque limiter 16b may be connected to the separation roller 16a so as to be substantially incorporated into the separation roller 16a. Incidentally, a structure as illustrated in FIG. 12B can be used as a means for retarding rotation that applies a driving force in a direction opposite to a sheet conveying direction to a separation roller. Referring to FIG. 12B, a torque limiter 17b is fixed to a driving shaft 17c of a separation roller 17a so that the separation roller 17a and the torque limiter 17b are connected to each other on the same shaft.
The above-described torque limiters 16b and 17b need to be configured as a separate structure. Since the torque limiters 16b and 17b are connected on the same shaft as the separation rollers 16a and 17a, or are incorporated into the separation rollers 16a and 17a, a combined structure is not symmetric in the longitudinal direction, increasing the overall size and production cost of a sheet feeding apparatus. As described above, the conventional sheet feeding apparatus employing the above-described torque limiter cannot be produced in a small size and at a low cost.
In addition, since a sheet conveying path for feeding out a sheet is not symmetric in the longitudinal direction, the leading end of the sheet is blocked midway in the sheet conveying path, or the sheet conveying operation is not properly performed due to the difference of conveying resistance on the left and right sides of the sheet conveying path. Therefore, it is necessary to devise means for guiding sheets such as a sheet guiding surface or wall.
Japanese Patent Application Laid-Open No. H08-026513 describes a torque limiter in which a tubular friction member is fixed on the outer circumference of a rotary member, and a tubular member is fitted to the outer surface of the rotary member. A plurality of sliding members is fitted to the friction member through a window portion of the tubular member. The plurality of sliding members is brought into pressure contact with the outer circumferential surface of the friction member by a spring member. Japanese Patent Application Laid-Open No. H07-269589 describes a torque limiter in which a main body member is fitted to a member for outputting power through a friction member, and a twisted-coil spring is applied to the outside of the power outputting member to fasten the power outputting member. The power outputting member is provided with a cylindrical fitting portion having a coil spring installed on the outer circumference. In the fitting portion, a plurality of slit groove portions are formed which extends in the axial direction from a flange portion and is opened at one end thereof.
The structures discussed in Japanese Patent Application Laid-Open Nos. H08-026513 and H07-269589 are difficult to produce in a small size and at a low cost because the torque limiter is configured as a separate structure independent from that of a sheet conveying roller.